In semiconductor production, wafers are coated with photoresist during the production process. The photoresist passes first through an exposure process and then a development process. In these processes, it is patterned for subsequent process steps. For production-related reasons, slightly more photoresist becomes deposited in the edge region of the wafer than in the middle of the wafer. An “edge bead” is thereby formed. The photoresist at the edge of the wafer, and the edge bead, can lead to contamination of production machinery and to the creation of defects on the wafer in subsequent process steps.
To eliminate these effects, an edge bead removal (EBR) process is performed. Errors in the width of the edge bead removal derive from inaccurate alignment of the corresponding edge bead removal devices relative to the wafer. Inaccurate alignment of the illumination devices relative to the wafer upon exposure of the photoresist constitutes a further error source. Edge bead removal over too wide an edge region results in a decrease in the usable wafer area, and thus in a loss of chip production. Insufficient edge bead removal can result, in the edge region of the wafer, in contamination of the subsequently applied resist layers or other features. Incomplete bead removal in the edge region can also, upon handling of the wafer by means of handling tools which, for example, engage into the edge region of the wafer, lead to impurities, e.g. abraded material, which cause contamination of the rest of the wafer surface and diminished quality in downstream process steps. Since the productivity of the production process is diminished in such cases, edge bead removal (along with many other defects) must be continuously monitored during the production process. The edge bead removal width is monitored, and a check is made as to whether edge bead removal has in fact taken place.
Devices are known which, by image recognition, detect a wide variety of features on the surface of a wafer. In this context, the wafer is illuminated in bright-field fashion and scanned with a camera (matrix camera or linear camera).
One such inspection machine of KLA-Tencor Corporation is described in the article “Lithography Defects: Reducing and Managing Yield Killers Through Photo Cell Monitoring,” by Ingrid Peterson, Gay Thompson, Tony DiBiase, and Scott Ashkenaz, Spring 2000, Yield Management Solutions. The wafer inspection device described therein operates with an incident-light illumination device that examines low-contrast microdefects using bright-field illumination.
EP 0 647 827 B 1 discloses a system for measuring the thickness of a thin-layer structure, in which light is irradiated onto the surface to be examined and interference rings are detected in the light reflected from the surface and evaluated using a computer, utilizing predefined learned patterns to determine the layer thickness.
In the known apparatuses for wafer inspection, the quality of the edge bead removal cannot easily be determined and assessed.
It is moreover difficult to make a distinction between the edge bead removal (EBR) and the other edges present in the image. These other edges derive from previous process steps. In bright-field illumination, all the edges are different in terms of color or grayscale value. Since the different edges also intersect or overlap in some cases, the color or grayscale value of the edges also changes. It is therefore very difficult or impossible to filter out the edge bead removal in this fashion using an image processing system. Even a visual inspection by an observer produces no better results, since the human eye also cannot manage to allocate the various edges and observed colors or grayscale values to the various process steps.